Final answer:
Long bone growth during infancy and youth is exclusively through interstitial growth of the epiphyseal plates.
Step-by-step explanation:
The growth of long bones during infancy and youth is exclusively through interstitial growth of the epiphyseal plates. The epiphyseal plates are responsible for longitudinal bone growth, where chondrocytes in the plate divide and one cell moves towards the diaphysis while the other remains near the epiphysis. This results in the replacement of cartilage with bone on the diaphyseal side of the plate, leading to bone lengthening.